A package of this basic kind is e.g. disclosed in WO2009/074721A1. This disclosure relates to a method for forming a package and a package with the shape of a container being formed of a cardboard blank. The package is provided with a bottom and sidewalls extending upwards from the bottom. At the end stage of compression moulding, a rim of moulding material extending sideways is moulded to the upper part of the sidewalls when the mould is kept closed in such a way that a mould cavity corresponding to the rim is formed by moving a counter surface inside the mould backwards from a surface opposite to it, after which the moulding material is introduced into the mould cavity. This provides the package with a horizontally extending flange which may be used to glue or heat seal a plastic or paper lid onto the upper surface of the horizontally extending flange.
The package of WO2009/074721A1 is an improvement over the basic kind of tray-shaped packages that have been used for relatively long time for, amongst others, pre-cooked ready-to-heat meals.
A similar kind of package is also disclosed in US2004/262322A1. This document discloses e.g. a method in which the rim of a tray-like package is moulded of a plastic material separately. It discloses a design, in which the blanks are joined together at their edges with moulded plastic material, after which the plastic rim is moulded to the upper edges of said sidewalls, to form a flange extending outwardly from them and encircling the package.
The most common so-called tray packages for keeping food is often formed from a cardboard blank shaped with a horizontal rim or flange extending from the upper end of the sidewalls and encircling the package. The packages are often closed, or even hermetically sealed, with a lid formed of a sheet of plastic or paper. The upper surface of the cardboard blank or the lower surface of the lid normally comprises a suitable coating, by means of which the lid can be attached to the package securely. For example, it is common to use heat-sealable plastic materials as coatings on the cardboard blank or on the lid, or to form the lid of a heat-sealable plastic material, to attach the lid to the package. Examples of food packages and package materials used therein are mentioned e.g. in documents WO 2003/033258, EP 1 289 856, WO 2000/21854, and U.S. Pat. No. 5,425,972.
The above mentioned WO2009/074721A1 was amongst others developed to take into account that sometimes there is a need to provide the edge flange with a material that is suitable for fixing the lid but is not necessarily required in the middle part, that is, inside the package, or that is even unsuitable as a material inside the package. Another problem may also be posed by insufficient mechanical properties, for example insufficient rigidity, of the edge flange. The choice of materials for the inner part of the package should not excessively determine the materials for the edge flange.
However, it has been found in some cases the rigidity is still not satisfactory, e.g. it is especially pronounced when a snap lock is to be fastened to the rim. It has also been found that the injection moulding tool and injection valve system imposes limitations concerning the different kinds of rims that may be shaped.